1. Field of the Invention
The present invention relates to a straight tube LED lamp using a semiconductor light-emitting element as a light source and a lighting device incorporating such an LED lamp, in particular, to the heat radiation from a substrate on which the semiconductor light-emitting element is mounted and a power supply board.
2. Description of the Related Art
For the purpose of energy saving, straight tube lamps using semiconductor optical elements such as LEDs have increasingly replaced fluorescent lamps and candescent lamps. The semiconductor optical elements excel a fluorescent lamp in terms of longevity and no blinking and flickers over time.
Japanese Patent Application Publication No. 2011-28946 (Reference 1) discloses an LED lighting device which comprises a long semi-cylindrical exothermic chassis, a semi-circular translucent cover attached on a planar portion of the chassis and having almost the same outer diameter as that of the semi-cylinder of the chassis, and a long substrate closely attached to the outer face of the planar portion and on which semiconductor optical elements as LEDs are mounted with a predetermined interval along the length. It contains a power supply board and lines for the LEDs in a hollow of the chassis.
Further, Japanese Patent Application Publication No. 2011-113876 (Reference 2) discloses an LED lighting device which comprises a transparent or semi-transparent tube with an opening on a circumference, a heat sink fitting into the opening, and semiconductor optical elements as LEDs mounted inside the tube. The heat sink is of a hollow structure directly or indirectly connected to the LEDs.
Further, Japanese Patent Application Publication No. 2011-210669 (Reference 3) discloses an LED lighting device which comprises an aluminum lamp fitting and a lighting portion detachable from the lamp fitting. The lighting portion includes a long aluminum LED substrate on which semiconductor optical elements are disposed, a long aluminum base of which the LED substrate is attached to a bottom surface, and a long cover attached to the base to expose at least the top surface of the base and cover the LED substrate. The lamp fitting includes a power supply circuit connected to an external power source to light the LEDs. The top surface of the base contacts the bottom surface of the lamp fitting while the lighting portion is mounted in the lamp fitting. The LED substrate is attached onto the base by an adhesive with a thermal conductivity.
There is a problem with the LED lighting device disclosed in Reference 1 that the temperature of the sealed chassis tends to rise highly due to the power supply board and wiring contained in the chassis. Especially, the parts such as a condenser and a coil on the power supply board may exceed 120 degrees, causing a difference in temperature along the length of the chassis with or without the power supply board. The LED substrate is attached closely to the chassis on the opposite side of the power supply board. The temperature of the LEDs may exceed 90 degrees but is lower than that of the power supply board so that the heat therefrom is transferred to the substrate. Thus, there may be unevenness in the temperature of the substrate due to the heat radiation from the power supply board such that the part close to the power supply board is higher in temperature than the part away therefrom. An incidence may occur that over time some LEDs greatly affected from the heat become burned out while the rest of the LEDs are lighting in a single straight tube lamp.
In Reference 2 the LEDs are directly or indirectly connected to the heat sink for heat transfer. The LEDs are accordingly prevented from rising in temperature and receives less thermal load. However, it lacks a direct current source so that the lamp fitting needs to include an AC-DC converter to supply direct currents to the LEDs from a commercial power source or a power source supplied from a stabilizer. This requires an additional electric work and incurs extra costs. If an existing straight tube lamp as fluorescent lamp is not replaced with the LED type and remains in the lamp fitting, erroneously connecting it to the commercial power source or the power source from the stabilizer and the flow of alternating currents may cause a fire or electrification.
In Reference 3 the LED substrate having a power source is attached to the base as a heat sink by a thermal adhesive. However, since it is integrated with the lamp fitting, an electric work is needed to replace an existing fluorescent lamp with this LED straight tube lamp, incurring extra costs. Further, to replace the LED substrate, the lamp fitting has to be disassembled, taking a longer time for maintenance. The power source is contained in a housing which is greatly larger than the diameter of the straight tube LED lamp. Cooling effects by airflow are expected but this device cannot replace the fluorescent lamp.